The present invention relates to a vehicle-body front structure, and specifically relates to a vehicle-body front structure which comprises front side frames extending in a vehicle longitudinal direction at right and left sides of an engine room, projecting forward from a vehicle-compartment portion, and a sub frame provided at a bottom portion of the engine room, the sub frame comprising a body portion which is comprised of right-and-left side portions and a front side portion and tower portions, the right-and-left side portions constituting attachment portions of lower arms of a front-wheel suspension, the front side portion interconnecting the right-and-left side portions in a vehicle width direction, the tower portions being provided to stand substantially vertically at right and left sides of the body portion and fastened to respective lower face portions of the front side frames at respective upper end portions thereof, the right-and-left side portions being fastened to a bottom portion forward of the vehicle-compartment portion at respective rear portions thereof.
Herein, the front side frames may not provide the resistance that is large enough against a collision load when a vehicle frontal collision occurs. Accordingly, a vehicle-body front structure, in which the sub frame is provided below the front side frames and therefore both the front side frames and the sub frame can receive the collision load so as to absorb collision energy, has been proposed recently.
A structure disclosed in Japanese Laid-Open Publication No. 2005-271811 is known as an example of the above-described vehicle-body front structure. That is, a sub frame is provided below front side frames, this sub frame extends forward from a floor frame jointly-fixed to a lower face of a floor panel, and a front end of the sub frame is located at a specified level below the front side frames. Herein, this sub frame is configured such that a bending portion is formed at a middle portion, in a longitudinal direction, of the sub frame so that a front portion of the sub frame forward of this bending portion rises sharply. Therefore, there is a problem in that when the vehicle frontal collision occurs, the above-described bending portion may deform downward, so that improper buckling may happen to the sub frame.
Herein, a structure shown in FIG. 16 as a comparative example may be considered in order to solve the above-described problem. That is, as shown in this figure, this structure comprises front side frames 100 which extend in a vehicle longitudinal direction at right and left sides of an engine room (FIG. 16 shows a right-side part only), projecting forward from a vehicle-compartment portion and a sub frame 105 which is provided at a bottom portion of the engine room, the sub frame 105 comprising a body portion 103 which is comprised of right-and-left side portions 101 and a front side portion 102 and tower portions 104, the right-and-left side portions 101 constituting attachment portions of lower arms of a front-wheel suspension, the front side portion 102 interconnecting the right-and-left side portions 101 in a vehicle width direction, the tower portions 104 being provided to stand substantially vertically at right and left sides of the body portion 103 and fastened to respective lower face portions of the front side frames 100 at their respective upper end portions 104a, the right-and-left side portions 101 being fastened to a bottom portion (not illustrated) forward of the vehicle-compartment portion at their respective rear portions.
Further, an extension member 106 is welded to a middle portion, in a vertical direction, of a pole portion 104b of the above-described tower portion 104 at its base portion, and this extension member 106 is configured to extend forward from the pole portion 104b and have a bending portion for inward-bending 106a which bends inward, in the vehicle width direction, only at its longitudinally-middle portion. According to this structure, when the vehicle frontal collision occurs, the front side frame 100 and the extension member 106 may jointly receive the vehicle-frontal collision load, so that the extension member 106 may bend inward, in the vehicle width direction, as shown by imaginary lines in FIG. 16.
In this comparative example shown in FIG. 16, however, there is a problem in that when the collision load is inputted, the stress may concentrate at the base portion (a so-called root portion) of the extension member 106 and consequently the welding portion may be apart, so that the extension member 106 may get broken at its base portion as shown by the imaginary lines in FIG. 16.